1. Field of the Invention
The present invention relates to a process for obtaining thin-layer passive circuits with resistive lines of different layer resistances as well as the passive circuit made by such a process.
2. Description of the Prior Art
There is a known need for producing on the same substrate of a passive thing-layer circuit two different types of resistive lines having different layer resistances.
Recent techniques teach producing resistive lines having greater and lesser resistivity by superimposing on the thin layers an additional layer of a second material having less resistivity.
In accordance with a first known technique the material with greater resistivity is a ceramic-metal alloy (CERMET) and the material with less resistivity is a nickel-chromium alloy.
In accordance with another known technique the material with less resistivity is a titanium-palladium alloy and the material with greater resistivity is tantalium nitride (Ta.sub.2 N).
An example of the last-mentioned technique is described in the article entitled: "A NEW MULTIPLE SHEET RESISTANCE TECHNOLOGY FOR THIN FILM CIRCUITS", by David Norwood, Proceedings of the 1985 International Symposium on Microelectronics--Anaheim, Calif., U.S.A. 11-14 Nov. 1985, pp. 281-286.
This article teaches a variant, applied to the standard process of Ta.sub.2 N thin film resistor manufacturing, consisting of an additional photoprocessing step to obtain lower value resistors from titanium and palladium layers (TiPd resistors); e.g. in line 2 of abstract, and in column 2 of page 282.
The main drawback of this process, as is explicitly admitted in the article, is that the manufactured TiPd resistors exhibit, within the range of utilization, an excessively high temperature coefficient (TCR) and poor temperature stability of the TCR, as shown in FIGS. 8 and 9, on page 285 thereof. The temperature sensitivity of the TiPd low value resistors is mainly due to the palladium layer, as this element has a TCR of 3770 PPM per degree Celsius (PPM/.degree.C.) in its elemental forms; however the Ti and Ta.sub.2 N underlayers reduce the TCR to some extent.
This higher TCR appears to be the limiting factor in using this process.
A second drawback of Norwood's process is that the TiPd resistors obtained have a positive TCR while the Ta.sub.2 N resistors have a negative TCR; consequently the temperature variation amplifies the value of the resistive gap between the two types of resistor.
Both of the above described prior art techniques have the drawback of requiring very complicated, long and costly production processes. In particular, they require a photoengraving step after deposit of the first material.
The object of the present invention is to provide a passive thin-layer circuit with resistive lines having different layer resistances by a simpler, more rapid and economical process more profitable than processes in accordance with prior art techniques.